The= and == operators have the same functionality.
They are implemented by the exact same class.

system.registerGenericUDF("=", GenericUDFOPEqual.class);
system.registerGenericUDF("==", GenericUDFOPEqual.class);
FunctionRegistry.java

+----------+---------------------+----------------------------------------------------------------+
| Operator |    Operand types    |                          Description                           |
+----------+---------------------+----------------------------------------------------------------+
| A = B    | All primitive types | TRUE if expression A is equal to expression B otherwise FALSE. |
| A == B   | All primitive types | Synonym for the = operator.                                    |
+----------+---------------------+----------------------------------------------------------------+